ICP-OES: A Versatile and Sensitive Method to Analyze Trace Metals

Inductively coupled plasma-optical emission spectrometry (ICP-OES) is one of two current gold standard methods for identifying and quantifying trace metal elements in liquids and substrates. Compared to its sister method employing mass spectrometry (ICP-MS), ICP-OES is more versatile and accessible. It is lower-cost, less training- and expertise-intensive in terms of collection, detection, and analysis, and it has a more robust ability to handle multiple elements in complex sample types.

Using inert argon gas and high frequency current at the tip of a torch tube, ICP-OES ionizes gas within a powerful electromagnetic field. This creates a high-temperature (> 10,000 Kelvin), electron-dense plasma within which a sample vaporizes. It then enters a narrow aperture, and elements undergo excitation. When elements fall back to their ground energy states, a detector receives spectral emissions from the spectrometer that identify them by the signature positions of their photon rays. Emission intensities act as surrogates for quantities of excited atoms at a given wavelength, resulting in calculated contributions of trace elements, in contrast to direct quantification by mass in ICP-MS. Instrumentation for ICP-OES can be optimized for sequential detection by using, for instance, a Czerny-Turner monochromator and photomultiplier detection system. For simultaneous detection, an echelle cross-dispersing spectrometer and a charge coupled device enable full-spectrum detection of over 70 elements in a few seconds. The versatility to efficiently and sensitively achieve both types of detection is a distinguishing feature of ICP-OES. Leading suppliers offer multiple configurations of each type, in addition to a la carte instrumentation to optimize ignition, detection, and analysis to the end user’s needs. For instance, nebulizers and spray chambers can be essential appendages to mitigate the confounding effects of volatility and viscosity in complex organic matrices.

ICP-OES in place of AAS

The identification and analysis of trace elements have evolved over more than a half century, from comparatively crude colorimetric and volumetric methods that were eclipsed by more sensitive and accurate techniques, particularly atomic absorption spectrometry (AAS). Although analogous in principle to AAS, the use of inert gas in ICP techniques creates a much higher ignition temperature. This results in more efficient excitation and eliminates most oxide/nitride byproducts generated by air/acetylene-based ignition in AAS methods. These improvements result in greater stability, a wider linear range within an analytic curve, and an ability to detect more elements more quickly at lower minimum limits. Therefore, AAS is increasingly a legacy method used for detection and analysis of comparatively high-concentration metals within purer matrices.

ICP-OES across industries

There are some general parameters for deciding what technology to use for trace metal detection. The most sensitive instrumentation with the greatest dynamic linear range is ICP-MS, so that when parts-per-trillion levels of detection are necessary, ICP-MS is the best option. These measurements are especially relevant when examining only one element, particularly one that is very toxic with a corresponding low regulatory minimum contaminant level (MCL), such as lead, arsenic, or mercury, whose catastrophic effects in drinking water supplies have caused their MCLs to be consistently lowered to infinitesimal levels. However, the sensitivity of ICP-MS often requires the user to employ serial dilutions to stay within a linear analytical range and can preclude the ability to glean concentrations from soil or other matrices that may supersede water in long-term environmental impact. In contrast, the flexibility of ICP-OES to simultaneously test for multiple elements at concentrations across multiple orders of magnitude can allow one to discern the contributions of toxic elements, such as lead, and elements essential for life in small amounts, such as nickel or selenium.

The optimal combination of versatility and sensitivity inherent in ICP-OES marks it as the best platform from which to analyze this spectrum of matrices.

Standard methods defined by the EPA also give guidance to using the appropriate technology. Methods 200.8 and 200.7 cover ICP-MS and ICP-OES, respectively, but with limitations imposed on each. For example, although both methods and technologies can be used to address Clean Water Act and Safe Drinking Water Act concerns, MS is not approved for measuring most minerals in drinking water. Hence, ICP-OES is often a more efficient and reliable option for drinking water, and for other “high-matrix” samples that contain unknowns and dissolved solids, as is often the case with ground and wastewater, soil, environmental, and industrial samples. Therefore, some other prominent uses of ICP-OES include detection of contaminants in petroleum refinement processes, and forensic analyses of trace metals in soil at crime scenes.

The importance of ICP-OES today

The importance of using the most accurate and accessible instrumentation and methodology for trace metal analysis can be highlighted by the following scenario. The epidemic of violent crime that arose seemingly everywhere, all at once in the 1970s, peaked by the early 1990s and then rapidly subsided, bore the counter-intuitive hallmark of being molecular in nature. Its arc replicated the rise and fall of tetraethyl lead developed and commercialized by General Motors as a gasoline additive, with a 23-year lag corresponding to the time it took for future criminals to mature. Moreover, statistical and demographic studies looking back as far as the early twentieth century have shown unequivocal links between municipal lead exposure and delinquency at the scale of individual neighborhoods. 

Structurally similar to calcium, lead blocks a range of neuronal pathways dependent on calmodulin-based signal transmission. Therefore, lead exposure is a major driver of developmental maladies, from attention deficit disorders to significantly lowered IQs. Lead is also notoriously persistent, and has shifted in its prevalence: from acidic drinking water in nineteenth century municipal pipes; to performance-enhanced gasoline and subsequently the soil beside highways, whipped by summer breezes into ingestible aerosols; to chipping paint on worn window ledges; and sometimes back to drinking water again. The optimal combination of versatility and sensitivity inherent in ICP-OES marks it as the best platform from which to analyze this spectrum of matrices.